[ZODB-Dev] Re: ZODB not saving sometimes

[Laurence Rowe]

Andreas Jung wrote:
> 
> 
> --On 22. Juni 2008 08:49:32 -0700 tsmiller <tsmiller at gnixterhouse.com> 
> wrote:
> 
>>
>> Gary,
>> I have been using the ZODB for about a year and a half with a bookstore
>> application.  I am just now about ready to put it out on the internet for
>> people to use.  I have had the same problem with saving data.  I have
>> tried alot of things.  But I have never gotten the database to save
>> consistently.  I can create an x number of records one right after the
>> other that uses the exact same code to save them, and it is likely that
>> all of them will save perfectly except one - or maybe two.
> 
> We have never seen that - except with badly written application code. 
> Commiting a transaction should always commit the data. That's the sense 
> of a transaction system. The only reason I can imagine causing such a 
> failure:
> bare try..except with in your code suppressing ZODB conflict errors.

The other likely cause of this is modifying non-persistent sub objects 
and not setting _p_changed = True on the parent persistent object. e.g:

     >>> dbroot['a_list'] = [1, 2, 3]
     >>> transaction.commit()
     >>> a_list = dbroot['a_list']
     >>> a_list.append(4)
     >>> transaction.commit()

The second commit actually has no effect as the persistence machinary 
has not been notified that the object has changed. This is not 
immediately apparent though as the 'live' shows what you expect:

     >>> a_list
     [1, 2, 3, 4]

And if a later transaction also modifies the persistent object, then all 
of the data is saved.

To avoid this, avoid using mutable, non-persistent types for storage in 
the ZODB, replace lists and dicts with PersistentList and PersistentMapping.

Laurence